1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle having an engine and one or more electric machines and, in particular, to controlling torque transmitted to the drive wheels when the vehicle is located on a hill and the engine is shutdown.
2. Description of the Prior Art
A powershift transmission is a geared mechanism that includes no torque converter, but instead employs two input clutches driveably connected to an engine crankshaft. A powershift transmission produces multiple gear ratios in forward and reverse drive and transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
During a vehicle creep condition with an increasing uphill road grade, the vehicle will eventually come to a stand-still condition due to the increased road load. At the point of stand-still for the given road grade, the vehicle is required to be stationary unless the driver tips into the accelerator pedal. While the vehicle is “holding” it should not roll backward at any point unless the road grade further increases beyond the stand-still point. The road grade at which vehicle stand-still (“hill-hold”) should occur is specified as a vehicle performance requirement and can vary by manufacturer or vehicle model.
In a conventional vehicle equipped with a powershift transmission, because there is no torque converter, the necessary engine torque for vehicle hill-hold is transferred to the vehicle wheels by slipping the appropriate input clutch.
The vehicle can be held on a hill using a powershift transmission for only a limited period due to degradation in clutch torque transfer capacity as the thermal limits of the clutch are reached while the clutch is slipping. The thermal capacity of the clutch also limits hill-holding to lower road grades.
If the vehicle rolls back during a hill-hold condition due to degrading clutch torque transfer with increasing clutch temperature, the vehicle operator will eventually tip-in to the accelerator pedal or begin using the wheel brakes to keep vehicle stationary on the hill.
Furthermore, the vehicle can roll-back when the driver tips-in to the accelerator pedal when transitioning from a hill-hold condition to a forward moving condition on a grade.
When the operator tips-in to accelerator pedal in order to keep the vehicle in a hill-hold, the required clutch torque capacity is increased while the vehicle is stopped, which further degrades clutch durability since the clutch is also slipping.
In a hybrid electric vehicle equipped with a powershift transmission, vehicle hill-hold can be challenging when considering the added powertrain operating modes, particularly conditions in which the engine is shut down.
There is a need in the industry for a control strategy that uses, during engine shutdown conditions, additional power or torque sources and the function of the transmission to provide vehicle hill-hold.